Hugo Salgado from Chile managed to come up with a plausible solution: the spike is due to both DLV and the root trust anchor being used concurrently. I consulted NLnet Labs to confirm this theory and they answered the following:

“DLV lookups are done using recursion. So if a TLD is in DLV (and has no DS in the root zone) every validatable lookup for that domain counts twice because the DLV is signed as well. As soon as the TLD moves its trust anchor to the root zone, this double counting goes away. In the period the data is for this happened for both .se as well as .gov”

Since there was no easy way to send Hugo a real pie he accepted a token ASCII art one, the proof is below: